Media for recording and reading information are ubiquitous in electronic equipment such as computers, digital cameras, and the like. One type of such media is a flexible data storage disk having a magnetic recording layer disposed over a non-magnetic substrate layer. Examples of these flexible disks include the type commonly referred to as a "floppy disk," a ZIP disk (manufactured by the assignee of the present invention), and the like.
Magnetic disks typically have an "embedded servo," wherein data regions having data and control signal regions having servo information for controlling the position of the magnetic head, are recorded alternately to constitute a recording track. During a process commonly known as "servo-writing," servo information is embedded into the magnetic layer of the magnetic disk. The servo information typically defines substantially concentric, circular tracks. Often, servo information is embedded in a sequence of quadrature servo patterns. Each pattern typically comprises four servo pulses, or bursts, each of which is offset from the neighboring servo burst. Such an embedded servo type disk has an advantage with respect to data recording density, as compared with magnetic disk apparatus in which the track on which the data are recorded and the tracks on which the servo signals are recorded are formed separately on the magnetic disk.
In a conventional embedded servo type magnetic disk apparatus, a read/write control signal, namely the index sector pulse (ISP) signal, is produced in response to the servo information from the control signal of the track, and the read data are transferred to an external unit (e.g. a host computer) in response to the generation of the ISP signal (concerning the "data transfer" operation).
In the conventional magnetic disk apparatus employing embedded servo, one index sector pulse signal functioning as the base point for the read/write control is outputted in response to the servo information which has been read out from the servo (control signal) region of the recorded disk plane. That is to say, in response to signal edges of the servo information S.sub.n, S.sub.(n+1) and S.sub.(n+2), the index sector pulse signals ISP.sub.n, ISP.sub.(n+1), and ISP.sub.(n+2) are formed. In response to these ISP signals, the read/write controls for the corresponding data ID.sub.n, ID.sub.(n+1), ID.sub.(n+2) are performed. In other words, a single read/write operation is carried out with respect to a single servo region. This process is described in greater detail in U.S. Pat. No. 5,313,340.
To determine head location relative to a track centerline, a head or transducer measures the signal from each burst. A position error signal ("PES") is determined by comparing the amplitude of the signals read from neighboring bursts. The PES is proportional to the difference between the signal amplitudes of the neighboring bursts, divided by the sum of their signal amplitudes. Thus, the PES represents the offset distance between the head and track centerline as defined by the servo information embedded in the disk. The PES is then used as part of a closed loop servo system to correct the position of the head with respect to the track.
There are known methods for determining the rotational position of a magnetic medium but these methods typically require a relatively significant amount of overhead to be reliable. That is, these methods typically require a relatively high number of bits on the medium dedicated to rotational positional determination. Thus, there is a need in the art for a highly reliable, low overhead method for determining the rotational position of a magnetic storage medium.